Corrosion inhibiting pigment composition

ABSTRACT

A corrosion inhibitor pigment composition is based a solid solution of (Ni 2+  +Co 2+ )-bis-hydrogen cyanamide.

This is a continuation-in-part of application Ser. No. 08/155,586 filedon Nov. 19, 1993, now U.S. Pat. No. 5,487,779.

FIELD OF THE INVENTION

This invention relates to corrosion inhibiting pigment compositions.More particularly, it relates to such compositions having particularapplicability to mirror backing formulations which are based on solidsolutions of salt derivatives of hydrogen cyanamide formed with certaintransition metals such as nickel and cobalt.

BACKGROUND OF THE INVENTION

Silver layers on mirrors are extremely sensitive to the presence andcorrosive action of airborne contaminants, e.g., salt particles typicalin coastal regions, or H₂ S, NH₃ or acidic contaminants which are alwayspresent in domestic or urban environments. These contaminants, in thepresence of moisture, are able to promote oxidative processes whichoccur according to the reaction Ag-e→Ag⁺ and result in the corrosivedisintegration of the mirror's reflective layer. Specialized corrosionpreventive coating systems, known as "mirror backing" coatings areapplied in order to extend a silver mirror's service life.

It is known that non-pigmented protective coatings formulated without acorrosion inhibitor pigment component, exhibit limited anti-corrosionprotection and consequently do not provide long term service life.Therefore, corrosion inhibitor pigments are critically importantfunctional components of mirror coating systems and basically determinethe useful service life of the protected silver surface.

It is known in the prior art to use various lead salts, such as leadcyanamide as corrosion retardant pigment components of mirror backingformulations. The specific corrosion retardant activity displayed bythese compounds on Ag surfaces is attributed to the presence of leadspecies and perhaps to their S²⁻ scavenging capacity. Due to theexcellent performance of mirror backing formulations containing leadcompounds, such systems have been widely used for decades by the mirrormanufacturing industry. Efforts to develop lead-free alternatives havinganticorrosive activity for silver have been spurred by environmentalconcerns.

U.S. Pat. No. 4,707,405/1987 (Evans et al.) discloses the use of metalderivatives of hydrogen cyanamide other than lead cyanamide as corrosionretardant pigment components in mirror backing protective coatings.However that patent limits the concept exclusively to cyanamides formedby Group IIA and IIB elements such as magnesium, calcium and zinc.

There are several chemical and physical properties which a pigment gradeproduct must possess in order to function as a component of a paintformulation and/or mirror backing protective coating. Among the requiredcharacteristics are limited water solubility, moderate alkalinity andcompatibility with resins and solvents used in such formulations.Specifically the capacity to interact with, and provide inhibitiveactivity against substrate specific electro-chemical oxidant processes,which promote corrosive decomposition of Ag surfaces, are the mostimportant, and are determined by the pigment's chemical composition andstructure. In this sense the disclosures of the above identified U.S.Patent leave a need for further improved pigment systems for use inmirror backing formulations.

Zinc cyanamide is known for its valuable pigmentary proprieties and itsapplicability as anticorrosive pigment in primer formulationsrecommended, i.e., for steel surface protection. In my U.S. Pat. No.5,176,894 issued Jan. 5, 1993, I disclosed a pigment grade zinccyanamide which meets the quality requirements for a mirror backprotective coating, i.e., high assay and absence of soluble salts.

It is well known in the chemical literature that hydrogen cyanamide, adi-basic acid, forms neutral, as well as basic and acidic saltderivatives with numerous metals, inclusive of the Group IA, IIAelements and some metals of the first, second and third transitionseries, among others with Co²⁺ and Ni²⁺.

It is important to note, however, that Ni²⁺ and Co²⁺ are the onlyspecies known to form the bis-hydrogen cyanamide structures symbolizedby Me(HNCN)₂ (Me²⁺ =Ni²⁺, Co²⁺) where the Me/NCN stoichiometrical ratiois 1:2.

Considering that the ═N--C.tbd.N moiety of any cyanamide compound, dueto its characteristic structure, is likely to generate theelectrochemically active inhibitor species (by interacting with moisturein situ in the protective coating) accountable for substrate specificcorrosion preventive activity on silver, it becomes evident that suchderivatives characterized by "bis" structure should be preferablyemployed as inhibitor pigments. In support of this observation it willbe noted that the theoretical value in weight % of the "NCN" content forZnNCN is only 37.9 compared to 58.1 for Ni or Co bis-hydrogen cyanamide.

Bernard, et al., [Compt. Rend. Ser. C 262(3), 282-4 (1966) report thatNi²⁺ and Co²⁺ species form cyanamide derivatives of bis-hydrogencyanamide structure and disclose a relevant preparation procedure basedon the precipitation reaction involving hexamine-nickelate or cobaltatespecies and H₂ NCN in ammoniacal medium, as follows:

    Me SO.sub.4 +6NH.sub.4 OH→[Me(NH.sub.3).sub.6 ].sup.2+ SO.sub.4.sup.2- +6H.sub.2 O                               1.

    [Me(NH.sub.3).sub.6 ].sup.2+ SO.sub.4.sup.2- +2H.sub.2 NCN⃡Me(HNCN).sub.2.H.sub.2 O↓+(NH.sub.4).sub.2 SO.sub.4 +4NH.sub.4 OH                                             2.

where Me=Co²⁺ or Ni²⁺

Typically the procedure is carried out by introducing H₂ NCN intoammoniacal solution of hexamine-nickelate or --cobaltate and bysubsequent agitation of the system for 18 hours at pH=7.5. A similarprocedure is disclosed, specified exclusively for Ni²⁺ cyanamide inExample 4 of Japanese Patent Nr. SHO 29-8020/12.07.54. The process isperformed in one hour by simultaneously introducing nickel sulfatesolution and ammonia gas into hydrogen cyanamide solution, at 25°-30° C.and by keeping the pH of the system at 7.5 to 8.5.

The NiSO₄ /H₂ NCN=1:1 molar ratio, recommended, quite surprisingly, bythis Japanese patent, fails to consider the bis-hydrogen cyanamidestructure of the intended product and represents a basicstoichiometrical error (See reaction 2.), which results in particularlylow yield (of about 53% in Ni(HNCN)₂.H₂ O) based on the disclosed value,and the correspondingly high amount (practically 50%) of unconvertedNiSO₄ dissolved and lost in the process water.

As expected, the disclosed value of the obtained product's nitrogencontent (33.2% N) is consistent with bis-hydrogen cyanamide composition,however of a relatively poor quality, which is a direct consequence, aswell, of the employed inadequate NiSO₄ /H₂ NCN molar ratio. For the samereason under the final conditions of the process (absence of H₂ NCN,high Ni²⁺ concentration, pH˜8.0) basic divalent nickel salts alsoprecipitate which, by subsequent dehydration are converted into darkcolored, inactive inclusions in the final products.

In addition to the above-exemplified shortcomings, there are inherentlimitations of the manufacturing procedures known by the prior art, allspecifically related to the precipitation reaction involvinghexamine-nickelate or --cobaltate and H₂ NCN, respectively to theemployment of ammonia as pH control reagent. Beside the inconveniencecaused by the volatility of NH₃ at the recommended pH value, whichrequire the employment of protective technologies, the followinglimitations are observed:

1. Reaction 2, a typical process of precipitation involving dissolvedhexamine nickelate or cobaltate species, reaches an equilibrium whichobstructs the completion of the direct reaction (basically the formationof the product by precipitation) to the desirable extent, even atsubstantial stoichiometrical excesses of H₂ NCN. This undesirablecharacteristic of the reaction system is a direct consequence of thepresence of ammonium salts, soluble by-products formed according toreaction 2, capable to prevent the complete precipitation of Ni²⁺ orCo²⁺ species as cyanamides. Thus the yield of the process issubstantially reduced and the resulting process water (mother liquor andwash water) contains large amounts of irrecuperable Ni²⁺ species as wellas undesirable ammonium salts.

2. As the complete removal of the soluble by-products (usuallyaccomplished by extensive washing) is the critical phase of anycorrosion retardant pigment manufacturing process which essentiallydetermines the quality of the final product, processes that use ammoniafor pH control, for aforementioned reasons, result in large amounts ofnon-recyclable, environmentally hazardous process water with high Ni²⁺or Co²⁺ contents.

SUMMARY OF THE INVENTION

Consistent with the above disclosed considerations, it is a principalobject of the present invention to provide inhibitor pigment gradeproducts and coprecipitated pigment compositions characterized byhis-hydrogen cyanamide structures, or containing such products, andprocedures to manufacture the same. The pigment compositions producedaccording to the subsequently disclosed procedures provide corrosioninhibitive activity, specifically on silver substrates, and are suitableto be employed in "mirror backing" paint and coating formulations.

Coprecipitated pigment compositions comprising solid solutions of Ni²⁺+Co²⁺ bis-hydrogen cyanamides and various transition metal cyanamides;i.e., Pb²⁺, Cd²⁺, Cu²⁺, Ag⁺, etc. can be obtained according to thesubsequently disclosed procedures. However, without any intent to limitthe applicability of the present invention, the relevant Examples arelimited to Co²⁺, Ni²⁺, and Zn²⁺ coprecipitated cyanamides.

As subsequently disclosed, solid solutions of Ni²⁺ +Co²⁺ bis-hydrogencyanamide, or compositions comprising such derivatives coprecipitatedwith transition metal cyanamides, more specifically zinc cyanamide, areproduced at pigment grade quality (high assay, soluble saltcontaminant-free conditions) according to an environmentally compatibleprocedure which results in heavy metal-free waste waters.

It was observed pursuant to the present invention that Co-bis-hydrogencyanamide and Ni-bis-hydrogen cyanamide, if produced by coprecipitationas disclosed hereinafter, in any weight ratios, form solid solutions of(cobalt and nickel)-bis-hydrogen cyanamides. In contrast, with solidphysical mixtures or blends which consist of distinct, identifiablephase components which are separable by mechanical procedures, solidsolutions display monophasic behavior in that all constituents areuniformly dispersed at an ionic or molecular level, and consequentlysuch systems behave as distinct chemical entities.

Briefly summarized, the invention provides pigment grade solid solutionsof Ni²⁺ +CO²⁺ bis-hydrogen cyanamide derivatives with enhanced corrosioninhibitive activity on silver. The solid solutions are formed bycoprecipitating, in situ, mixtures of Ni(OH)₂ and Co(OH)₂, and reactingthe same with a stoichiometrical excess of H₂ NCN.

Coprecipitated pigment compositions containing bis-hydrogen cyanamidederivatives and zinc cyanamide are also produced by introducingdispersed and hydrated ZnO into the reaction system and reacting it withH₂ NCN at appropriate stoichiometrical ratios.

BRIEF DESCRIPTION OF DRAWINGS

IR spectra characterizing Ni²⁺ and Co²⁺ bis-hydrogen cyanamides andsolid solutions of (Co²⁺ /Ni²⁺) bis-hydrogen cyanamides are shown inFIG. 1, and,

IR spectra of coprecipitated pigment compositions based on ZnNCN and theaforementioned bis-hydrogen cyanamide derivatives in 10/1 molar ratio,are presented in FIG. 2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The above presented limitations, which are characteristic to themanufacturing procedures known by the prior art, are eliminatedaccording to the present invention by producing high quality pigmentgrade Ni²⁺ and Co²⁺ bis-hydrogen cyanamides and solid solutions of Ni²+Co²⁺ bis-hydrogen cyanamides based on the heterogeneous reactioninvolving insoluble and freshly coprecipitated Ni(OH)₂ and Co(OH)₂ andH₂ NCN, as follows:

    MeSO.sub.4 +2NaOH→Me(OH).sub.2 ↓+Na2SO.sub.4 3.

    Me(OH).sub.2 ↓+2H.sub.2 NCN→Me(HNCN).sub.2.H.sub.2 O↓4.

where Me=Ni²⁺ or Co²⁺

Such pigment grade quality solid solutions of Ni²⁺ +Co²⁺ bis-hydrogencyanamides typically are characterized by high N (or cyanamide) content,virtual absence of insoluble contaminants and light green or light browncolor, respectively.

Both, Ni²⁺ and Co²⁺ species are characterized by similar chemicalbehavior under the conditions of reaction 3 and 4; both Co(OH)₂ andNi(OH)₂ are insoluble in excess of NaOH or in the presence of solubleby-products of reaction 3, and if freshly precipitated, reactinstantaneously and quantitatively with H₂ NCN.

As above indicated, in contrast to reaction 2, reaction 4 is adiffusionally controlled heterogeneous process which takes place at theMe(OH)₂ liquid interface and converts the freshly precipitated solidprecursor into the solid final product. It is observed that the reactionplausibly occurs by nucleophile substitution, where the .sup.(-) HNCNanion is actually the reactive species, formed by dissociation favoredin alkaline medium, as follows:

    H.sub.2 NCN+OH.sup.- ⃡HNCN.sup.- +H.sub.2 O    5.

As a consequence of the above presented considerations, the rate ofreaction 4 is determined by the magnitude of the specific surface areaof the hydroxide phase, the .sup.(-) HNCN concentration and, mostimportantly, the alkalinity of the medium. It will be observed, however,that the effect of the medium's alkalinity on the reaction rate is acomplex one; it does favorably regulate the concentration of the.sup.(-) HNCN anion but also promotes the well known dimerization of H₂NCN, in this case an undesirable process, competitive with reaction 4.

Under the conditions determined by reaction 4, processes could occur, aswell, which are undesirable in reference to the object of the presentinvention and adversely affect the final products' quality.

In this respect, it will be observed that under mild basic conditions,Co²⁺ species specifically tend to turn into insoluble basic salts ofCo(OH)x type (where x=NO₃ -, SO₄ 2-), which although are readilyconvertible into Co(OH)₂ with appropriate excess of NaOH, do notthemselves react according to reaction 4.

Equally undesirable is the tendency of freshly precipitated Co(OH)₂ toundergo slow oxidation on exposure to air, yielding dark colored andnon-reactive Co(OH)₃, according to the following reaction:

    4Co(OH).sub.2 +O.sub.2 +2H.sub.2 O→4Co(OH).sub.3 ↓6.

Such coprecipitated non-reactive species are converted by dehydrationinto darkly colored oxides, which, as inclusion contaminants, degradethe typically light color and reduce the cyanamide content of the finalproducts.

The incomplete conversion according to reaction 4 of the Co(OH)₂ orNi(OH)₂ reactive precursor species, due to less than optimal ⁻ HNCNconcentration in the reaction medium, as well, adversely affects thefinal product's quality.

The occurrence of these undesirable side reactions are preventedaccording to the present invention by selecting appropriate processparameters, as subsequently disclosed.

In this respect, it is observed, that reactions 3 and 4 according to thepresent invention are preferable carried out by instantaneously reactingNi(OH)₂ and/or Co(OH)₂, freshly precipitated in situ in the reactionmedium, with an appropriate excess of H₂ NCN.

During precipitation, which is performed in 15 to 30 minutes, thealkalinity and temperature of the reaction medium is kept at pH=8 to9.0, preferably 8.5, and 15°-25° C., respectively.

The above specified optimal reaction conditions are maintained bysimultaneous and individual introduction of the raw materials, atappropriate rates, into the reaction medium. The method of thisinvention also includes the further steps of filtering the reactionproduct, washing the filtered product with water to a substantiallysalt-free condition and subsequently drying the product.

It was observed pursuant to the present invention that Co-bis-hydrogencyanamide and Ni-bis-hydrogen cyanamide, if produced by coprecipitationas disclosed hereinafter, in any weight ratios, form solid solutions ofcobalt/nickel -bis-hydrogen cyanamides. These solid solutions displaymonophasic behavior by containing all constituents uniformly dispersedat an ionic/molecular level. Such systems, thus, can be regarded asdistinct chemical entities.

Formally, the chemical composition and structure of solid solutionsformed by Co- and Ni-bis-hydrogen cyanamides, are represented by

(yCo+zNi)(HNCN)₂, where y+z=1 and 0<y, z<1

In support of this statement the IR spectra of Co(HNCN)₂, Ni(HNCN)₂ and(yCo+zNi)(HNCN)₂, where y=z, are presented. (See FIG. 1.) As it can beseen, all the above cited spectra are identical in all details (see bandat 3292 cm⁻¹) and more specifically so in respect of the fine structureor relative intensities displayed by the component bands of the2000-2300 cm⁻¹, 1100-1200 cm⁻¹ and 560-620 cm⁻¹ characteristicabsorption bands.

The existence of solid solutions of (yCo+zNi)(HNCN)₂, where z+y=1 andO<y, z<1; related synthesis procedures and the above disclosed identityof pertinent IR spectra are facts observed pursuant to the presentinvention and unknown in the prior art.

Considering the state of knowledge regarding the chemistry of variousmetal derivatives of cyanamides, the above specified experimental factsare unexpected, as well. In that sense is important to observe thatcyanamide derivatives of different metals are generally not known by theart to form solid solutions.

As for the identity (equivalency) of IR spectra of Co--Ni--, and solidsolutions of (Co+Ni)-bis-hydrogen cyanamides, as above disclosed, itdoes constitute an anomaly, for metal-cyanamides characteristicabsorption band's exact position (generally situated at 3200-3500, 2000to 2200 and 560-680 cm⁻¹) and fine structure are dependent on the metalcations' chemical identity. See the details regarding the IR spectrum ofZnNCN in my U.S. Pat. No. 5,378,446, as well as in FIG. 2.

Synthesis procedures and analytical data are presented subsequently,pertinent to the following:

Co(HNCN)₂ and Ni(HNCN)₂ ;

Physical mixtures of ZnNCN+Co(HNCN)₂ and ZnNCN+Ni(HNCN)₂ ;

Solid solutions of (yCo+zNi)(HNCN)₂ where y=z;

Physical mixtures of ZnNCN and solid solutions of (yCo+zNi)(HNCN)₂,where y=z;]

It will be noted, that for reasons of simplicity and with no intentionto limit the applicability of the present invention, the data disclosedbelow are given exclusively in reference to (yCo+zNi)(HNCN)₂, where y=z,which is only one of numerous solid solutions possible in the Co(HNCN)₂/Ni(HNCN)₂ system.

With respect to the IR spectra presented in FIGS. 1 and 2, the narrow,intense band at 3292 cm⁻¹ is noted, which constitutes a characteristicfeature, assignable perhaps to the N--H bond of the --N(H)CN or hydrogencyanamide moiety. The presence of a relatively intense triplet band at1100-1200 and 560-620 cm⁻¹, appear to be also characteristic to thebis-hydrogen-cyanamide structure.

The triplet band situated roughly at 2000-2300 cm⁻¹ region is known tobe typical for metal derivatives of cyanamide. As above stated andnoticeable on FIG. 2, its exact position and the component bands'relative intensity are dependent on the pertinent cations' chemicalidentity. The IR spectrum presented on FIG. 2 relates to thecoprecipitated physical mixture (similar to Example 7), characterized bythe phase composition formula of

[ZnNCN/(yCo+zNi)(HNCN)₂ ].(H₂ O)_(x), where y=z, and x=0 to 1. Itdisplays absorption bands characteristic of both distinct solid phasecomponents and it can be regarded as a composite resulted bysuperposition of ZnNCN's IR spectrum (for reference see FIG. 1 of U.S.Pat. No. 5,176,894) and the appropriate one presented in FIG. 1. Notableare the absorption bands situated at approximately 679 cm⁻¹ and 2033cm⁻¹ are characteristic to ZnNCN.

EXAMPLE 1

In order to exemplify the aforementioned inherent limitations of themanufacturing procedures known by the prior art, Ni(HNCN)₂.(H₂ O)_(x),where x=0-1 was produced following basically the recommendationsdisclosed in Example 4 of the above-identified Japanese Patent, withsome exceptions, as follows:

adequate Ni²⁺ /H₂ NCN=1:2,2 molar ratio was applied, correspondent tothe bis-hydrogen cyanamide structure of the final product

concentrated aqueous NH₃ solution was employed as pH control reagent

all three reagent solutions involved were simultaneously introduced intothe reactor at appropriate rate and the total volume of the aqueousphase was comparatively reduced to practical values

The process was carried out in 30 minutes by simultaneously introducingSolutions (A), (B) and (C) containing the appropriate amounts of rawmaterials [(A) -394.1 g. or 1.5 moles of NiSO₄.6H₂ O in 900 ml.; (B)-138.6 g. or 3.3 moles of H₂ NCN in 500 ml.; (C) -57.0 g. or 3.3 molesof NH₃ in 500 ml.] into intensively stirred H₂ O at 25°-30° C.

A temporary stoichiometrical excess of H₂ NCN was initially generatedand kept during the precipitation process, by introducing "ab initio"10% of Solution (B) into the reactor. Subsequently, all during theprecipitation process, Solutions (A) and (B) were introduced atidentical rates; however, the delivery rate of (C), the ammoniasolution, was adjusted as required to keep the reaction medium's pH=7.5to 8.5 as recommended by the aforementioned Japanese patent.

The precipitation process was completed by fixing the reaction system'spH=8.5 and stirring for an additional 30 minutes. Subsequently, thevivid green precipitate obtained was filtered, washed to salt-freeconditions and dried at 70° C. for 12 hours. The experimental resultsare presented in Table 1.

                  TABLE 1                                                         ______________________________________                                        Measured Parameters Found Values                                              ______________________________________                                        Product's appearance                                                                              Vivid green powder                                        Yield               191.5 g.; 80.5%                                           Ni as Ni %          40.0                                                      N as N %            33.3                                                      Ni (HNCN).sub.2 %   83.7 calculated, based                                                        on N %                                                    H.sub.2 O %         9.8 calculated                                            Non-soluble impurities,                                                                           6.5 calculated                                            possible NiO %                                                                N/Ni ratio          3.49                                                      Recovered Process Water:                                                      Volume 2320 ml.                                                               Ni.sup.2+  Concentration 5.78 g./l.                                           Total Ni.sup.2+  Content 13.4 g.                                              Recovered wash water:                                                         Volume 6500 ml.                                                               Total Ni.sup.2+  Content 2.67 g.                                              Total amount of Ni.sup.2+  lost in                                            process water and wash water 16.07 g.                                         Calculated amount of lost Ni(HNCN).sub.2 : 43.44 g.;                          (18.2%)                                                                       ______________________________________                                    

It is remarkable that 18% of the final product is not recoverable, beingdissolved in the process water, even at optimal Ni/H₂ NCN=1:2.2 molarratio. As above indicated, the low yield and related consequences aredue to the fact that the precipitation reaction involving solublehexamine nickelate or cobaltate species respectively is based on theemployment of ammonia as the pH control reagent.

The practical embodiment of the present invention is illustrated in 6examples as follows:

EXAMPLE 2

Pigment grade cobalt bis-hydrogen cyanamide with enhanced corrosionpreventive activity symbolized by the structural formula Co(HNCN)₂.(H₂O)_(x) (where x=0-1) was produced according to the following procedure:

A cobalt salt solution (A) was prepared by solubilizing 1 mole (263.06g.) technical grade CoSO₄.6H₂ O (from Hydrite Chemical Co. of Wisconsin)in 700 ml. water by stirring it at ambient temperature. Water was addedto adjust the volume of the Co²⁺ solution to 800 ml.

Solution (B) containing 2.2 moles of hydrogen cyanamide was prepared byintroducing 185.0 g. of stabilized aqueous hydrogen cyanamide solutionof 50% by weight concentration (available from Cyanamide Canada Inc.under the trade name of Cyanamide-50) into 100 ml. water of normaltemperature and by adjusting the solution's final volume to 500 ml.

A sodium hydroxide solution (C) of approximately 10% concentrationcontaining 2 moles of NaOH was prepared by introducing 160 g. technicalgrade caustic solution of 50% by weight into 400 ml. cold water, coolingit subsequently to ambient temperature and by adjusting the volume to800 ml.

In order to prepare pigment grade cobalt bis-hydrogen cyanamide, thepreviously prepared (A), (B) and (C) solutions were introducedsimultaneously in approximately 30 minutes (applies to A and C), atidentical volumetric rate of approximately 25-30 ml/minute into 400 ml.intensively stirred water at normal temperature.

Substantial stoichiometrical excess of hydrogen cyanamide was assured inthe reaction system by carefully keeping the reactant solutions'delivery rate constant all during the precipitation as well as byproviding some initial excess of that reagent.

The slurry's pH was monitored accurately and kept rigorously betweenpH=8.0-9.0 and preferably at pH=8.5 by adjusting the NaOH (C) solution'sdelivery rate accordingly.

The precipitation was completed by stirring the slurry unheated for onehour at pH=8.5 and the process was completed by heating and stirring theslurry at 40° C.±5° C. for 30 minutes.

Subsequently, the product was washed to an essentially soluble salt-freecondition by repeated sedimentation of the slurry, decantation, freshwater introduction and reslurrying until a conductivity of 300micromhos/cm of the supernatant was reached; at that stage byCyanamide-50 addition, a concentration of 0.1% hydrogen cyanamide wasgenerated and the slurry was subsequently stirred for 15-30 minutes.

After solid-liquid separation, the pigment grade cobalt bis-hydrogencyanamide was obtained by drying it at 55°-65° C. for 12 hours to 4-8%H₂ O content and pulverizing it to fineness of 100% +230 mesh.

It is important to note that the mother liquor resulting from the firstdecantation of the slurry, as well as the wash water generated, wascolorless and contained only 0.5-1 mg/l dissolved CO²⁺.

Total amount of pigment grade product recovered was 154 g.

Analytical data obtained on dried product typical for pigment gradecobalt bis-hydrogen cyanamide produced according to the above describedprocedure are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                        Analyzed or Tested Value                                                                      Test Procedure                                                Parameter       Found       Applied                                           ______________________________________                                        Appearance      Light brown                                                                   powder                                                        Co as Co %      40.0        Complexometry                                     N as N %        37.2        Kjeldahl                                          Co bis-hydrogen 93.6        Calculated                                        cyanamide %                 value based on                                                                N %                                               H.sub.2 O %     5.2         Calculated                                        N/Co ratio      3.91                                                          Non-soluble impurities                                                                        1.2         Calculated                                        probable Co.sub.2 O.sub.3 %                                                   Conductivity    200 ± 25 ASTM D-281-31                                     (micromhos/cm)                                                                pH              9.0-9.2     ASTM D-1208-                                                                  84/6.1.1                                          Specific Gravity                                                                              2.23        ASTM D-113-84                                     Oil Absorption Lbs/100 Lbs                                                                    27-30       ASTM D-281-31                                     Yield           154.0 g.;                                                                     96.9% as                                                                      monohydrate                                                   Total amount of process water                                                                          Approx. 10                                           (mother liquor & wash water)                                                  recovered                                                                     l.                                                                            Co.sup.2+  concentration in process water                                                              <1                                                   mg/l.                                                                         ______________________________________                                    

EXAMPLE 3

Pigment grade nickel bis-hydrogen cyanamide (symbolized by thestructural formula Ni(HNCN)₂.(H₂ O)_(x) (where x=0-1) with enhancedcorrosion preventive activity suitable to be employed in "mirrorbacking" paint formulations was produced according to the procedurepursuant to Example 2, except that Solution (A) was prepared bydissolving 1 mole (262.7) of technical grade NiSO₄.6H₂ O in a similarfashion, as presented in Example 2.

Total amount of pigment grade product recovered was 155.0 g.

It is observed that the mother liquor recovered from the firstdecantation of the slurry, as well as the wash water generated, wascolorless and contained 0.5-1 mg/l Ni²⁺.

Analytical data (obtained on dried product) typical for the pigmentgrade nickel bis-hydrogen cyanamide produced according to the procedureabove described are presented in Table 3.

                  TABLE 3                                                         ______________________________________                                        Analyzed or Tested                                                            Procedure       Value       Test                                              Parameter       Found       Applied                                           ______________________________________                                        Appearance      Light green                                                                   powder                                                        Ni as Ni %      38.4        Complexometry                                     N as N %        36.3        Kjeldahl                                          Ni bis-hydrogen 91.2        Calculated                                        cyanamide %                 value based on                                                                N %                                               H.sub.2 O %     8.3         Calculated                                        N/Ni ratio      3.96                                                          Non-soluble impurities                                                                        ˜0.5                                                    probable NiO %                                                                Calculated                                                                    Conductivity    200 ± 25 ASTM D-281-31                                     (micromhos/cm)                                                                pH              9.0-9.3     ASTM D-1208-                                                                  84/6.1.1                                          Specific Gravity                                                                              2.22        ASTM D-113-84                                     Oil Absorption Lbs/100 Lbs                                                                    27-30       ASTM D-281-31                                     Yield           155.0 g.;                                                                     97.6% as                                                                      monohydrate                                                   Total amount of process water                                                                          9-10                                                 recovered                                                                     l.                                                                            Ni.sup.2+  concentration in process                                                                    <1 mg                                                water                                                                         Ni.sup.2+ /l.                                                                 ______________________________________                                    

EXAMPLE 4

Coprecipitated pigment grade Co--Ni bis-hydrogen cyanamide solidsolution of (yCo+zNi)(HNCN)₂.(H₂ O)_(x), where y=z and x=0 to 1,characterized by enhanced corrosion preventive activity, suitable to beemployed in mirror backing paint system formulations, was producedaccording to the procedure pursuant to Example 2, except that Solution(A) was prepared by dissolving 0.5 mole of each CoSO₄.6H₂ O (131.45 g.)and NiSO₄.6H₂ O (131.35 g.) in a similar fashion, as presented inExample 2.

Total amount of pigment grade solid solution recovered (before grinding)was 152.0 g.

It is observed that the mother liquor recovered from the firstdecantation of the slurry was, as well as the wash water generated,colorless and contained 0.5-1 mg/l. Co²⁺ and Ni²⁺.

Typical analytical data (obtained on dried product) characterizing thepigment grade solid solution obtained according to the above-presentedprocedure, are presented in Table 4.

                  TABLE 4                                                         ______________________________________                                        Analyzed or Tested                                                            Procedure       Value       Test                                              Parameter       Found       Applied                                           ______________________________________                                        Appearance      Light brown                                                                   powder                                                        (Ni & Co) %     38.6        Complexometry                                     N as N %        36.7        Kjeldahl                                          (Co + Ni) bis-hydrogen                                                                        92.3        Calculated                                        cyanamide %                 value based on                                                                N %                                               H.sub.2 O %     7.6         Calculated                                        N/(Co + Ni) ratio                                                                             3.99                                                          Non-soluble impurities                                                                        <0.2                                                          (probable NiO + Co.sub.2 O.sub.3) %                                           Calculated                                                                    Conductivity    200 ± 50 ASTM D-281-31                                     (micromhos/cm)                                                                pH              8.6-8.9     ASTM D-1208-                                                                  84/6.1.1                                          Specific Gravity                                                                              2.23        ASTM D-113-84                                     Oil Absorption, 27-30       ASTM D-281-31                                     Lbs/100 Lbs.                                                                  Yield           152.0 g.;                                                                     95.7% as                                                                      monohydrate                                                   Total amount of process water                                                                           10-11                                               recovered                                                                     l.                                                                            Ni.sup.2+  + Co.sup.2+  concentration in process                                                        <1                                                  water                                                                         mg/l.                                                                         ______________________________________                                    

EXAMPLE 5

A coprecipitated physical mixture of Ni²⁺ bis-hydrogen cyanamide/zinccyanamide pigment system with 1:10 molar ratio, characterized by a phasecomposition of [ZnNCN/0.1.Ni(HNCN)₂ ].(H₂ O)_(x), where x=0-1, andenhanced corrosion preventive activity on silver substrate andapplicable in mirror backing formulations was produced according to theprocedure, as follows:

A combination (A) of a soluble nickel salt solution-zinc oxidesuspension was prepared by introducing 0.1 moles of NiSO₄.6H₂ O (26.3g.) as 30% aqueous solution, into a previously prepared ZnO suspensionof 10-12%, containing 1.0 mole (81.4 g.) of well dispersed and hydratedhigh grade ZnO at normal temperature and by adjusting the total volumeto 600 ml.

The above-mentioned ZnO suspension was obtained by introducing, in smallincrements, 81.4. of finely ground ZnO (Azo 66 grade, with averageparticle size of 0.25 microns and 99.8% assay, manufactured by Asarco,Inc. was used) into 400 ml. intensively stirred hot water at 75°-80° C.and by cooling it to ambient temperature after one hour.

Solution (B) containing 1.32 moles of hydrogen cyanamide was prepared byintroducing 110.9 g. Cyanamide-50 into 250 ml. water and by completingthe solution's volume to 400 ml.

A sodium hydroxide solution (C) of approximately 16% concentration,containing 10.2 moles of NaOH was prepared by introducing 16.0 g.technical grade caustic solution of 50% by weight into 50 ml. coldwater, cooling it subsequently to normal temperature and by completingthe volume to 100 ml.

Coprecipitated physical mixture of Ni²⁺ bis-hydrogen cyanamide/zinccyanamide at 1:10 molar ratio was produced by simultaneous introduction,at identical delivery rate, of solutions (A) and (B) into 200 ml.intensively stirred water, at normal temperature, respectively byadjusting the delivery rate of solution (C) accordingly to keep thereaction system's pH=8-9.0, and preferably at pH=8.5, all during thereaction.

By keeping the delivery rate of Solution (A) and (B) rigorouslyidentical, substantial stoichiometrical excess of H₂ NCN was kept in thereaction system all during precipitation.

The reaction was completed by stirring for two hours and adjusting theslurry's pH periodically to pH=8.5 by diluted H₂ SO₄ addition, and byheating it at 40° C.±3° C. for 30 minutes. Subsequently, the resultantcoprecipitated product was processed in a similar fashion as presentedat Example 2.

The total amount of pigment grade coprecipitated product recovered was120.5 g. The resultant mother liquor and wash water (total amount of6.01) was colorless and contained less than 0.5 mg. Ni²⁺ /l. Analyticaldata relevant to such coprecipitated physical mixture are presented inTable 5.

                  TABLE 5                                                         ______________________________________                                        Analyzed or Tested                                                            Parameter            Found Values                                             ______________________________________                                        Appearance           White-faint green                                                             powder                                                   Zn as Zn %           52.82                                                    Ni as Ni %           4.95                                                     N as N %             23.94                                                    ZnNCN/0.1 Ni (HNCN).sub.2 %                                                                        71.2 - Calculated                                        Oxides (ZnO, NiO) %  21.6 - Calculated                                        H.sub.2 O %          7.2 - Calculated                                         Zn/Ni ratio          9.58                                                     N/(Zn + Ni) ratio    1.91                                                     Conductivity         350-600                                                  (micromhos/cm)                                                                pH                   8.0-8.5                                                  Specific Gravity     2.80                                                     ______________________________________                                    

EXAMPLE 6

A physical mixture of Co²⁺ bis-hydrogen cyanamide/zinc cyanamidecoprecipitated pigment system with 1:10 molar ratio characterized by aphase composition of

[ZnNCN/0.1.Co(HNCN)₂ ].(H₂ O)_(x), where x=0-1, applicable in mirrorbacking formulations, was produced in an identical manner as presentedin Example 5, except that instead of NiSO₄.6H₂ O, Solution (A) contained0.1 moles (26.3 g.) of technical grade CoSO₄.6H₂ O.

The resultant process water was colorless and contained less than 0.5mg. Co²⁺ /l. A total amount of 120.0 g. of coprecipitated product wasrecovered. The relevant analytical data are presented in Table 6.

                  TABLE 6                                                         ______________________________________                                        Analyzed or Tested                                                            Parameter            Found Values                                             ______________________________________                                        Appearance           Light brown                                                                   powder                                                   Zn as Zn %           52.95                                                    Co as Co %           4.86                                                     N as N %             24.21                                                    ZnNCN + 0.1 Co(HNCN).sub.2                                                                         72.0 - Calculated                                        H.sub.2 O %          6.4 - Calculated                                         Oxides (ZnO, Co.sub.2 O.sub.3) %                                                                   21.6 - Calculated                                        Zn/Co ratio          9.82                                                     N/(Zn + Co) ratio    1.93                                                     Conductivity         350-600                                                  (micromhos/cm)                                                                pH                   8.0-8.6                                                  Specific Gravity     2.78                                                     ______________________________________                                    

EXAMPLE 7

Pigment grade coprecipitated physical mixture containing a solidsolution of (Co+Ni)-bishydrogen cyanamide and ZnNCN in 1:10 molar ratio,characterized by a phase composition formula of[ZnNCN/0.1(yCo+zNi)(HNCN)₂ ].(H₂ O)_(x), where y=Z and x=0 to 1,suitable for mirror backing formulations, was prepared in identicalmanner as presented in Example 5, except that (A) contained 0.05 moles(13.1 g.) each of NiSO₄.6H₂ O and CoSO₄.6H₂ O.

A total amount of 120.5 g. of coprecipitated product was recovered. Theresulted process water was colorless and contained less than 0.5 mg./l.of Ni²⁺ and Co²⁺ species. The relevant analytical data are presented inTable 7.

                  TABLE 7                                                         ______________________________________                                        Analyzed or Tested                                                            Parameter              Found Values                                           ______________________________________                                        Appearance             Light brown                                                                   powder                                                 Zn as Zn %             52.87                                                  Ni and Co %            4.91                                                   N as N %               24.20                                                  ZnNCN + 0.1(Co + Ni)(HNCN).sub.2                                                                     72.0 - Calculated                                      Zn/(Co + Ni) ratio     9.68                                                   N/(Zn + Co + Ni) ratio 1.93                                                   H.sub.2 O %            7.2                                                    Oxides (ZnO, Co.sub.2 O.sub.3, NiO) %                                                                20.8                                                   Conductivity           350-600                                                (micromhos/cm)                                                                pH                     8.0-8.6                                                Specific Gravity       2.79                                                   ______________________________________                                    

The corrosion retardant materials of this invention can be incorporatedinto a mirror backing composition containing an organic film formingresin which is dispersed or dissolved in either water or an organicsolvent. The organic film forming resin should be one which iscompatible with the thin metallic mirror film and does not promotedeterioration and discoloration of the film. Thus, the resin should notcontain functional groups which are reactive with the metal. Preferredorganic film forming resins include alkyd resins, acrylic resins,acrylic modified alkyd resins, polyurethane oils, vinyl halide polymersor copolymers, epoxy melamine or urea resins, non-oil based urethane,phenolformaldehyde resins curable by air drying or baking, or any otherresin which is compatible to the metallic layers formed on mirrorbackings. Examples of solvents for such compositions include xylene,MIBK or toluene. The composition can also contain other components suchas catalysts, flow control agents, dryers, anti-settling agents, and thelike and mixtures thereof.

What is claimed is:
 1. A corrosion inhibitor pigment compositioncomprising a solid solution of (Ni²⁺ and Co²⁺)-bis-hydrogen cyanamide.2. A corrosion inhibitor pigment composition according to claim 1wherein said (Ni²⁺ and Co²⁺)-bis-hydrogen cyanamide is coprecipitatedwith zinc cyanamide.
 3. A corrosion inhibitor pigment compositionaccording to claim 1 wherein said solid solution is coprecipitated witha transition metal cyanamide selected from the group consisting of Pb²⁺,Cd²⁺, Cu²⁺, Ag⁺ and Zn²⁺.
 4. A composition according to claim 1 whereinsaid solid solution comprises approximately equal molar parts of cobaltand nickel.
 5. A mirror back coating composition for long termprevention of corrosion of silver coated thereon comprisingan organicresin polymer coating material carried in an aqueous or non-aqueousmedium; and, a corrosion inhibiting amount of a solid solution of (Ni²⁺and Co²⁺)-bis-hydrogen cyanamide interspersed in said medium.